PRESIDENT’S ADDRESS TO MANCHESTER MICRO. SOCIETY. 97 
specimens for microscopical investigation by the hardening, imbed- 
ding, clearing, and tinting processes. In 1831 Dr. Allen Thomson 
first introduced the method of section to the study of embryology 
by applying it to the investigation of an early state of development 
of the aorta of the bird. In 1840 Hanover, of Copenhagen, began 
to use chromic acid as a hardening agent ; and since that time later 
observers, with improved processes and better section instruments, 
have considerably extended the knowledge of this science and the 
art of investigation ; so that it is now possible to make very thin 
and clear sections, several hundreds to the inch, through the 
smallest embryo or the smallest egg, and that, as Dr. Allen Thom- 
son remarks, ‘‘notwithstanding the extreme delicacy of some of the 
parts and the inequality of their density, every one of the sections 
may be made to present a distinct and true view both of the 
microscopic histological characters and of the larger morphological 
relations of the parts observed.” 
It may be useful here to refer to some of the discoveries of the 
embryologist. It is known that all animals are produced from 
cells, and that in all animals except the Protozoa the reproducing 
cells are ova. All ova undergo segmentation or division of the 
germinal portion, by a process analogous to that of fission in the 
Protozoa, first into two halves, an upper and a lower, which event- 
ually give rise to two layers or membranes, an outer or ectoderm 
and an inner or endoderm. In the lowest animals produced from 
ova these two layers constitute the basis of the entire animal, and 
give rise to the animal and vegetative systems, or the external covering 
and the internal digestive cavity. But in the ova of animals of a 
higher order, a middle layer, or mesoderm, afterwards arises, which 
gives origin to the osseous, muscular and vascular systems. One 
of the earliest changes which supervene in the development of ver- 
tebrated animals is the formation of the rudimentary spine, the 
chorda dorsalis or notochord, which in the lowest form of fish, the 
Amphioxus or Lancelet, is persistent through life, that creature 
having neither skull nor vertebrz, but which, in the higher fish, 
gives rise to these bony structures of a simple form, and in the 
higher vertebrata to bones of more perfect development. Now it 
is a remarkable fact that this rudimentary spine, or notochord, was 
found by Kowalevsky in 1866, and since then by Kupfer and 
others, in the larve of the Ascidia, those marine, molluscoid 
animals having the form of a bottle with a double neck, inclosing 
the two external orifices of the body. They live either singly or in 
colonies, and are allied to the Polyzoa. They are, however, more 
highly organised, for they all possess a distinct heart in the form of 
a muscular tube without valves, which contracts rhythmically and 
propels the blood first in one direction (the respiratory) during a 
certain number of pulsations, and then, reversing its action, forces 
